1. Field of the Invention
The invention relates to enhanced oil recovery and somewhat more particularly to improved petroleum sulfonate products useful in enhanced oil recovery and a method of producing and utilizing such sulfonate products.
2. Prior Art
The petroleum industry has recognized for many years that only a small fraction of the original petroleum (i.e., crude oil) within a given reservoir is expelled by natural mechanisms. Further, it is recognized that conventional methods of supplementing natural recovery are relatively inefficient and economically unattractive. Typically, a reservoir may retain as much as half to two-thirds of the original petroleum therein, even after the application of currently available secondary recovery techniques, such as waterflooding. Conventionally, waterflooding involves injecting at least water, through one or more input wells to drive petroleum from the reservoir formation to a geometrically offset production well. Further improvements in oil recovery can be attained with certain enhanced oil recovery techniques, wherein oil recovery systems are formulated into micellar systems with surface-active agents, injected into input wells and driven or pushed through the reservoir formation to provide additional amounts of petroleum.
Surface-active agents or surfactants typically utilized for improving the efficiency of enhanced petroleum recovery methods must no only be economical but must also be compatible with the reservoir environment. Typically, such an environment includes localized higher temperatures, higher salt and/or polyvalent ion concentrations, adsorptive petroleum-bearing or petroleum-loving surfaces, minute pore spaces, etc., all of which potentially affect the surfactant and the petroleum recovery obtained by the use of such a surfactant. For example, petroleum retained within a reservoir after natural and/or secondary recovery processes are terminated, may be in the form of discontinuous globules or discrete droplets which are trapped within the pore spaces of the reservoir, along with connate, brackish or the like water at some particular temperature. Because of the normal interfacial tension between the reservoir petroleum and water in high, such discrete petroleum droplets are unable to sufficiently deform to pass through the narrow constrictions of the individual pore channels. Similarly, reservoir petroleum appears to have a greater affinity to the petroleum-bearing surfaces, i.e., rocks, sand, etc. than does the water so that any applied force merely pushes the water to an area of less pressure, i.e., a production well, while leaving the petroleum in place on the reservoir surfaces. When surface-active agents are formulated into an oil recovery system and injected into a reservoir, they function in numerous ways, one of which is to lower the interfacial tension between the flowable materials within a reservoir and permit the petroleum droplets to deform and flow with the surfactants in the flood water system toward a production well. It is generally conceded that the interfacial tension between the flood water system and the reservoir petroleum must be reduced to an order or less than about 0.1 dynes/cm for effective recovery. Surface-active agents must also be stable in the presence of higher temperatures encountered in at least some reservoirs and be stable in the presence of highly brackish water or polyvalent ions present in certain reservoirs and yet be able to "wash" the reservoir surfaces so as to release all or almost all petroleum adsorbed therein so as to achieve an economical and effective recovery process.
One of the more promising surface-active agents used in enhanced oil recovery are the petroleum sulfonates. Generally, these agents comprise the reaction products of a petroleum feed stock and a select material yielding a sulfo radical to the petroleum feed stock, i.e., oleum or gaseous or liquid SO.sub.3. Depending on many variables, such as the nature of the initial petroleum feed stock, the nature of the sulfo radical yielding material, the sulfonation reaction conditions selected, etc., the resulting petroleum sulfonates may be formulated with a wide variety of properties making them useful in enhanced oil recovery processes as well as in other fields of use, such as industrial surfactants, as blending agents for lubricating oils, as agricultural emulsifiers, dispersants, etc. However, economic and efficient production of petroleum sulfonates is difficult and the art is replete with various suggestions for achieving a more or less universally acceptable reaction process, even though most, if not all of the prior art processes leave much to be desired in terms of product control, economic availability of feed stocks, field of optimum surface-active properties, etc. In our earlier referenced disclosures, we teach a method of sulfonating petroleum oil feed stocks which provides a high yield of petroleum sulfonates and a method for formulating such sulfonates into enhanced oil recovery systems.
As a continuous part of our work in this field, we have conducted numerous core flood experiments on various crudes with various "slugs" containing various petroleum sulfonates in an effort to obtain optimum petroleum recovery under various reservoir conditions. As a part of these studies, we noticed that certain petroleum sulfonates tend to yield good petroleum recovery at certain conditions but yield different results at other conditions. We undertook to investigate the reason for such divergent results.
In enhanced oil recovery processes, the economics of a select recovery process and/or a surface-active agent, such as a petroleum sulfonate, are extremely stringent. Suggestions have been made in the art, that in order to minimize shipping and production costs, to either produce the petroleum sulfonate in-situ, as by injecting select reactive material directly into a reservoir, allowing a reaction to take place therein and then flooding the resulting reservoir with a water system to intermix with whatever reaction products are formed therein or to produce the petroleum sulfonates at the reservoir site and then formulate the resultant reaction product into a desired slug composition and inject such slug into the reservoir. However, neither of these suggestions have proven satisfactory since with the first suggestion, no control of reaction conditions or reaction products is available and with the second, insufficient quality control results, particularly since initial feed stocks, reaction conditions, etc. may vary from site to site. Further, the reaction products obtained from a typical petroleum sulfonation reaction tend to be non-homogeneous and unstable over a period of time so that the properties of such products vary. Typically, a petroleum sulfonation acidic reaction product mixture tends to separate into at least two phases, comprised of sulfonated sulfonatable components, non-sulfonated but solfonatable components and non-sulfonatable components. Upon standing, such acidic reaction product mixtures tend to change in composition and properties, apparently because some residual SO.sub.3 or degenerate specie thereof is present within the reaction products and gradually reacts with sulfonatable components therein, although other theories or explanations for this apparent instability may be equally valid. Nevertheless, such non-homogeneity and instability limits the extent of usefulness for such reaction products and often necessitates further processing before a final product is attained which is suitable for various industrial and commercial purposes, such as enhanced oil recovery in diverse reservoir environments.
U.S. Pat. No. 2,928,867 suggests that stable alkaryl sulfonates (i.e., dodecylbenzene sulfonates) may be prepared by sulfonating pure or relatively pure dodecylbenzene or postdodecylbenzene (a mixture of mono-and di-alkylbenzenes) with SO.sub.3 under sulfonation conditions, cooling the resulting alkaryl sulfonic acid, adding a small amount of water to the cooled sulfonic acid over a relatively brief period of time and then neutralizing the resultant sulfonic acid with caustic to obtain a pH-stable alkaryl sulfonate useful in detergent formulations. However, this prior art patent is silent as to enhanced oil recovery techniques or any surface-active agents useful in such oil recovery processes.